Method for forming polymer composite films using removable substrates

ABSTRACT

The invention is a method for forming polymer composite films using removable substrates comprising: 
     (a) forming a first dispersion of a first perfluorinated polymer containing sites convertible to ion exchange groups dispersed in a first dispersant having: a boiling point less than about 110° C.; a density of from about 1.55 to about 2.2; and a solubility parameter of from greater than about 7.1 to about 8.2 hildebrands; 
     (b) depositing the first dispersion onto a first removable substrate; 
     (c) removing the first dispersant from the first dispersion, thereby forming a first film; 
     (d) forming a second dispersion of a second perfluorinated polymer containing sites convertible to ion exchange groups and a second dispersant having: a boiling point less than about 110° C.; a density of from about 1.55 to about 2.2; and a solubility parameter of from greater than about 7.1 to about 8.2 hildebrands; 
     (e) depositing the second dispersion onto a second removable substrate; 
     (f) removing the second dispersant from the second dispersion, thereby forming a second film; 
     (g) bonding the first film to the second film; and 
     (h) removing the first and the second substrate. 
     Particularly preferred as a first and as a second dispersant is a compound represented by the general formula: 
     
         XCF.sub.2 --CYZX&#39; 
    
     wherein: 
     X is selected from the group consisting of F, Cl, Br, and I; 
     X&#39; is selected from the group consisting of Cl, Br, and I; 
     Y and Z are independently selected from the group consisting of H, F, Cl, Br, I and R&#39;; 
     R&#39; is selected from the group of perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms. 
     The most preferred first and second dispersant is 1,2-dibromotetrafluoroethane.

This is a continuation of application Ser. No. 739,943 filed May 31,1985, now abandoned.

The invention is a method for forming polymer composite films using aremovable substrate and particularly for forming ion exchange activecomposite membranes using a removable substrate.

BACKGROUND OF THE INVENTION

Ion exchange active fluoropolymer films have been widely used inindustry, particularly as ion exchange membranes in chlor-alkali cells.Such membranes are made from fluorinated polymers having sitesconvertible to ion exchange active groups on pendant groups from thepolymeric backbone.

Such polymers are usually thermoplastic and may be fabricated into filmsor sheets while in their molten form using mechanical extrusionequipment. However, such equipment is operated in the temperature regionnear the crystalline melting point of the polymer, which is commonlynear the decomposition temperature of some of the polymers. Thus,decomposition may be a problem when some polymers are formed into filmsby conventional methods. Likewise, it is difficult to make such polymersinto films thinner than about 10 microns using such techniques. Inaddition, it is difficult to make films of consistent thickness. Itwould be highly desirable to be able to make thin films having aconsistent thickness.

Forming membrane structures and support structures into multiple layersis the subject of several patents and applications including U.S. Pat.Nos. 3,925,135; 3,909,378; 3,770,567; and 4,341,605. However, thesemethods use complicated procedures and equipment including such thingsas vacuum manifolds, rolls and release media.

Prior art methods for fabricating films from perfluorinated polymershave been limited by the solubility of the polymers and thetemperature-dependent viscosity-shear rate behavior of the polymers. Toovercome these characteristics of perfluorinated carboxylic esterpolymers, workers have tried to swell the polymers using various typesof swelling agents and to reduce the fabrication temperatures of thepolymers to practical ranges by extraction. Extractions methods havebeen taught in, for example, U.S. Pat. No. 4,360,601. There, lowmolecular weight oligomers were removed from carboxylic ester polymers.Polymer "fluff" was extracted in a Soxhlet device at atmosphericpressure for 24 hours (see Examples 1 and 3 of U.S. Pat. No. 4,360,601).Such treatments has been found to make some fluorinated carboxylic estercopolymers more processible and operate more efficiently in achlor-alkali cell when in a hydrolyzed form. Such extractions modify thefabricated polymer article, for example, by forming grease of thepolymer as shown in Example 3 of U.S. Pat. No. 4,360,601.

In addition, such extractions seem to lower processing temperatures ofcarboxylic ester polymers after isolation. Isolation means separationfrom the polymerization latex by conventional methods of deactivatingthe surfactant such as freezing, heating, shearing, salting out or pHadjustment.

British Patent 1,286,859 teaches that highly polar organic "solvents"dissolve small amounts a fluorinated vinyl ether/tetrafluoroethylenecopolymer in its thermoplastic form. Thermoplastic form means thepolymer is in a form which can be molded or processed above sometransition temperature (such as the glass transition temperature or themelting point) without altering its chemical structure or composition.The patent teaches the use of the following materials "solvents":butanol, ethanol, N,N-dimethylacetamide, and N,N-dimethylaniline.

Similar approaches have been used to swell membranes in their ionicforms. Ionic forms of membranes are membranes which have been convertedfrom their thermoplastic form (--SO₂ F or --COOCH₃) to their ionic forms(--SO₃ M or --COOM) where M is H⁺, K⁺, Na⁺, or NH₄ ⁺ or other metal ion.

Prior art workers have used highly polar solvents or mixtures ofsolvents on substantially perfluorinated polymers and less polarsolvents on fluorinated polymers containing hydrocarbon components asco-monomers, ter-monomers or crosslinking agents.

However, each of the prior art methods for swelling, dispersing orextracting the polymers has certain shortcomings which are known tothose practicing the art. Polar solvents have the potential for waterabsorption or reactivity with the functional groups during subsequentfabrication operations, thus making poor coatings, films, etc. Highboiling solvents are difficult to remove and frequently exhibit toxic orflammability properties. Functional form (ionic forms) of the polymerscan react with solvents. (See Analytical Chem., 1982, Volume 54, pages1639-1641).

The more polar of the solvents such as methanol, butanol esters, andketones as used in U.S. Pat. No. 3,740,369; British Patent 1,286,859;and Chemical Abstracts 90:1697022 have high vapor pressures at ambientconditions, which is desirable for solvent removal; however, they tendto absorb water. Their water content is undesirable because it causesproblems in producing continuous coatings and films of hydrophobicpolymers. In addition, polar solvents frequently leave residues whichare incompatible with the polymers. Also, they frequently leave residueswhich are reactive during subsequent chemical or thermal operations ifthey are not subsequently removed.

Another approach taken by the prior art workers to form films fromfluoropolymers include the use of high molecular weight "solvents" whichhave been produced by halogenating vinyl ether monomers. (See BritishPatent 2,066,824A).

The swelling of the functional (ionic) forms of the fluoropolymers bypolar or hydrophilic agents has been known for some time. In addition,the solvent solubility parameters were compared to the swelling effectof 1200 equivalent weight Nafion ion exchange membrane (available fromE. I. DuPont Company) by Yeo at Brookhaven Laboratory (see Polymer,1980, Volume 21, page 432).

The swelling was found to be proportional to two different ranges of thesolubility parameter and a calculation was developed for optimizingratios of solvent mixtures. Ionic forms of functional fluoropolymers maybe treated in such a manner, however, the subsequent physical forming ormanipulation of the polymers into usable configurations by any thermaloperation is limited when the polymers are in the functional forms. Inaddition, non-ionic forms of polymers treated in this manner are alsolimited in the thermoplastic processing range by the stability of thefunctional group bonds.

Other solvation methods have used temperatures near the crystallinemelting points of the polymers being solvated, thus requiring eitherhigh boiling point "solvents" or high pressure vessels to maintain thesystem in a solid/liquid state. See Analytical Chem., 1982, Volume 54,pages 1639-1641.

Burrell states the theory of Bagley [J. Paint Tech., Volume 41, page 495(1969)] predicts a non-crystalline polymer will dissolve in a solvent ofsimilar solubility parameter without chemical similarity, association,or any intermolecular force. However, he fails to mention anything aboutthe solubility of polymers demonstrating crystallinity.

SUMMARY OF THE INVENTION

The invention is a method for forming polymer composite films usingremovable substrates comprising:

(a) forming a first dispersion of a first perfluorinated polymercontaining sites convertible to ion exchange groups and a firstdispersant having: a boiling point less than about 110° C.; a density offrom about 1.55 to about 2.97 grams per cubic centimeter; and asolubility parameter of from greater than about 7.1 to about 8.2hildebrands;

(b) depositing the first dispersion onto a first removable substrate;

(c) removing the first dispersion from the first dispersion, therebyforming a first film;

(d) forming a second dispersion of a second perfluorinated polymercontaining sites convertible to ion exchange groups and a seconddispersant having: a boiling point less than about 110° C.; a density offrom about 1.55 to about 2.97 grams per cubic centimeter; and asolubility parameter of from greater than about 7.1 to about 8.2hildebrands;

(e) depositing the second dispersion onto a second removable substrate;

(f) removing the second dispersant from the second dispersion, therebyforming a second film;

(g) bonding the first film to the second film; and

(h) removing the first and the second substrate.

Particularly preferred as a first and as a second dispersant is acompound represented by the general formula:

    XCF.sub.2 --CYZX'

wherein:

X is selected from the group consisting of F, Cl, Br, and I;

X' is selected from the group consisting of Cl, Br, and I;

Y and Z are independently selected from the group consisting of H, F,Cl, Br, I and R';

R' is selected from the group of perfluoroalkyl chloroperfluoroalkylradicals having from 1 to 6 carbon atoms.

The most preferred first and second dispersant is1,2-dibromotetrafluoroethane.

DETAILED DESCRIPTION OF THE INVENTION

Dispersion, as used herein, means a composition containing a treatingagent and a perfluorinated polymer containing sites convertible to ionexchange groups. The polymer is at least partially dissolved in thedispersant and is dispersed into the dispersant.

The present invention can be used to make ion exchange composite filmssuitable for use in electrolytic cells, fuel cells and gas or liquidpermeation units.

Non-ionic forms of perfluorinated polymers described in the followingpatents are suitable for use in the present invention: U.S. Pat. Nos.3,282,875; 3,909,378; 4,025,405; 4,065,366; 4,116,888; 4,123,336;4,126,588; 4,151,052; 4,176,215; 4,178,218; 4,192,725; 4,209,635;4,212,713; 4,251,333; 4,270,996; 4,329,435; 4,330,654; 4,337,137;4,337,211; 4,340,680; 4,357,218; 4,358,412; 4,358,545; 4,417,969;4,462,877; 4,470,889; and 4,478,695; European Patent Application0,027,009. These polymers usually have equivalent weights of from about500 to about 2000.

Particularly preferred for the formation of each layer of the compositefilms of the present invention are copolymers of monomer I with monomerII (as defined below). Optionally, a third type of monomer may becopolymerized with I and II.

The first type of monomer is represented by the general formula:

    CF.sub.2 =CZZ'                                             (I)

where:

Z and Z' are independently selected from the group consisting of --H,--Cl, --F, or CF₃.

The second monomer consists of one or more monomers selected fromcompounds represented by the general formula:

    Y--(CF.sub.2).sub.a --(CFR.sub.f).sub.b --(CFR'.sub.f).sub.c --O--[CF(CF.sub.2 X)--CF.sub.2 --O].sub.n --CF=CF.sub.2   (II)

where:

Y is selected from the group consisting of --SO₂ Z, --CN, --COZ and C(R³_(f))(R⁴ _(f))OH;

Z is I, Br, Cl, F, OR, or NR₁ R₂ ;

R is a branched or linear alkyl radical having from 1 to about 10 carbonatoms or an aryl radical;

R³ _(f) and R⁴ _(f) are independently selected from the group consistingof perfluoroalkyl radicals having from 1 to about 10 carbon atoms;

R₁ and R₂ are independently selected from the group consisting of H, abranched or linear alkyl radical having from 1 to about 10 carbon atomsor an aryl radical;

a is 0-6;

b is 0-6;

c is 0 or 1;

provided a+b+c is not equal to 0;

X is Cl, Br, F or mixtures thereof when n>1;

n is 0 to 6; and

R_(f) and R'_(f) are independently selected from the group consisting ofF, Cl, perfluoroalkyl radicals having from 1 to about 10 carbon atomsand fluorochloroalkyl radicals having from 1 to about 10 carbon atoms.

Particularly preferred is when Y is --SO₂ F or --COOCH₃ ; n is 0 or 1;R_(f) and R'_(f) are F; X is Cl or F; and a+b+c is 2 or 3.

Although the polymers of each layer can have the same or differentradicals for Y, the most preferred composite polymer is one where thepolymer of one layer has Y as --SO₂ F and the polymer of the other layerhas Y as --COOCH₃.

By composite films we mean film composed of two or more differentpolymers. These polymers may differ by type or concentration of sitesconvertible to ion exchange group. These different polymers are disposedin layers parallel to the film surface.

The third and optional monomer suitable is one or more monomers selectedfrom the compounds represented by the general formula:

    Y'--(CF.sub.2).sub.a' --(CFR.sub.f).sub.b' --(CFR'.sub.f).sub.c' --O--[CF(CF.sub.2 X')--CF.sub.2 --O].sub.n' --CF=CF.sub.2 (III)

where:

Y' is F, Cl or Br;

a' and b' are independently 0-3;

c' is 0 or 1;

provided a'+b'+c' is not equal to 0;

n' is 0-6;

R_(f) and R'_(f) are independently selected from the group consisting ofBr, Cl, F, perfluoroalkyl radicals having from about 1 to about 10carbon atoms, and chloroperfluoroalkyl radicals having from about 1 toabout 10 carbon atoms; and

X' is F, Cl, Br, or mixtures thereof when n'>1.

Conversion of Y to ion exchange groups is well known in the art andconsists of reaction with an alkaline solution.

The monomer FSO₂ CF₂ CF₂ OCF=CF₂ has a density of about 1.65 grams percubic centimeter and polytetrafluoroethylene has a density of about 2.2grams per cubic centimeter. A copolymer of this monomer withtetrafluoroethylene would, thus, have a density between the two values.

It has been discovered that certain perhalogenated dispersant have asurprising effect of dispersing the polymers, especially when thepolymers are in a finely divided state.

Dispersants suitable for use in the present invention should have thefollowing characteristics:

a boiling point less than about 110° C.;

a density of from about 1.55 to about 2.97 grams per cubic centimeter;

a solubility parameter of from greater than about 7.1 to about 8.2hildebrands.

It is desirable that the dispersant has a boiling point of from about30° C. to about 110° C. The ease of removal of the dispersant and thedegree of dispersant removal is important in the producing of variousfilms, coatings and the like, without residual dispersant; hence areasonable boiling point at atmospheric pressure allows convenienthandling at room conditions yet effective dispersant removal byatmospheric drying or mild warming.

It is desirable that the dispersant has a density of from about 1.55 toabout 2.97 grams per cubic centimeter. The polymers of the presentinvention have densities on the order of from about 1.55 to about 2.2grams per cubic centimeter. Primarily, the polymers have densities inthe range of from about 1.6 to about 2.2 grams per cubic centimeter.Dispersants of the present invention will therefore swell dissolve anddisperse small particles of this polymer, aided by the suspendingeffects of the similarity in densities.

The prior art did not balance density. They were interested in formingsolutions and solutions do not separate.

Solubility parameters are related to the cohesive energy density ofcompounds. Calculating solubility parameters is discussed in U.S. Pat.No. 4,348,310, the teachings of which are incorporated by reference forthe purposes of their teachings on solubility parameters.

It is desirable that the dispersant has a solubility parameter of fromgreater than about 7.1 to about 8.2 hildebrands. The similarity incohesive energy densities between the dispersant and the polymerdetermine the likelihood of dissolving, swelling and dispersing thepolymer in the dispersant.

It is preferable that the dispersant has a vapor pressure of up to about760 millimeters of mercury at the specified temperature limits at thepoint of dispersant removal. The dispersant should be convenientlyremoved without the necessity of higher temperatures or reducedpressures involving extended heating such as would be necessary in casessimilar to U.S. Pat. No. 3,692,569 or the examples in British Patent2,066,824A in which low pressures (300 millimeters) had to be employedas well as non-solvents to compensate for the higher boiling points andlow vapor pressures of the complex solvents.

It has been found that dispersants represented by the following generalformula are particularly preferred provided they also meet thecharacteristics discussed above (boiling point, density, and solubilityparameter):

    XCF.sub.2 --CYZ--X'

wherein:

X is selected from the group consisting of F, Cl, Br, and I;

X' is selected from the group consisting of Cl, Br, and I;

Y and Z are independently selected from the group consisting of H, F,Cl, Br, I and R';

R' is selected from the group of perfluoroalkyl radicals andchloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

The most preferred dispersants are 1,2-dibromotetrafluoroethane(commonly known as Freon 114 B 2)

    BrCF.sub.2 --CF.sub.2 Br and 1,2,3-trichlorotrifluoroethane (commonly known as Freon 113):

    ClF.sub.2 C--CCl.sub.2 F

Of these two dispersants, 1,2-dibromotetrafluoroethane is the mostpreferred dispersant. It has a boiling point of about 47.3° C., adensity of about 2.156 grams per cubic centimeter, and a solubilityparameter of about 7.2 hildebrands.

1,2-dibromotetrafluoroethane is thought to work particularly wellbecause, though not directly polar, it is highly polarizable. Thus, when1,2-dibromotetrafluoroethane is associated with a polar molecule, itselectron density shifts and causes it to behave as a polar molecule.Yet, when 1,2-dibromotetrafluoroethane is around a non-polar molecule,it behaves as a non-polar dispersant. Thus, 1,2-dibromotetrafluoroethanetends to dissolve the non-polar backbone of polytetrafluoroethylene andalso the polar, ion-exchange-containing pendant groups. Its solubilityparameter is calculated to be from 7.13 to 7.28 hildebrands.

It is surprising that an off-the-shelf, readily-available compound suchas 1,2-dibromotetrafluoroethane would act as a solvent for thefluoropolymers described above. It is even more surprising that1,2-dibromotetrafluoroethane happens to have a boiling point, a densityand a solubility parameter such that it is particularly suitable for useas a solvent/dispersant in the present invention.

In practicing the present invention, the polymer may be in any physicalform. However, it is preferably in the form of fine particles to speeddissolution and dispersion of the particles into the dispersant.Preferably, the particle size of the polymers is from about 0.01 micronsto about 840 microns. Most preferably, the particle size is less thanabout 250 microns.

To dissolve and disperse the polymer particles into the dispersant, thepolymer particles are placed in contact with the dispersant of choiceand intimately mixed. The polymer and the dispersant may be mixed by anyof several means including, but not limited to, shaking, stirring,milling or ultra sonic means. Thorough, intimate contact between thepolymer and the dispersant are needed for optimum dissolution anddispersion.

The polymers of the present invention are dissolved and dispersed intothe dispersants at concentrations ranging from about 0.1 to about 50weight percent of polymer to dispersant. At concentrations below about0.1 weight percent, there is insufficient polymer dissolved anddispersed to be effective as a medium for coating of articles or formingfilms within a reasonable number of repetitive operations. Conversely,at concentrations above about 50 weight percent there is sufficientpolymer present as a separate phase such that viable, coherent films andcoatings of uniform structure cannot be formed without particulateagglomerates, etc.

Preferably, the concentration of the polymer in the dispersant is fromabout 0.1 to about 20 weight percent. More preferably, the concentrationof the polymer in the dispersant is from about 0.3 to about 10 weightpercent. Most preferably, the concentration is from about 5 to about 15weight percent.

The dispersion of the polymer into the dispersant can be conducted atroom temperature conditions. However, the optimum dispersion effects arebest achieved at temperatures from about 10° C. to about 50° C. Attemperatures above about 50° C. the measures for dissolving anddispersing the polymer have to include pressure confinement for thepreferred dispersants or method of condensing the dispersants.Conversely, at temperatures below about 10° C. many of the polymers ofthe present invention are below their glass transition temperatures thuscausing their dispersions to be difficult to form at reasonableconditions of mixing, stirring, or grinding.

The dispersion of the polymers of the present invention into thedispersant are best conducted at atmospheric pressure. However,dispersion effects can be achieved at pressures from about 760 to about15,000 millimeters mercury or greater. At pressures below about 760millimeters mercury, the operation of the apparatus presents noadvantage in dissolving and dispersing polymers, rather hinderingpermeation into the polymers and thus preventing forming of thedispersions.

Conversely, pressures above about 760 millimeters mercury aid indissolving and dispersing polymers very little compared to thedifficulty and complexity of the operation. Experiments have shown thatat about 20 atmospheres the amount of polymer dissolved and dispersed inthe dispersant is not appreciably greater.

After the polymer dispersions of the present invention have been formed,they are fixed to other polymer films or substrates by sintering orcompression to fix the polymer from the dispersion to the substrate.

After coating the first dispersion onto the first substrate and coatingthe second dispersion onto the second substrate, it is possible tocontact the dispersions and form them into a fused composite film whilemaintaining them in contact. The two dispersions, when processed in thismanner, will tend to mingle to some extent.

The following methods are suitable for fixing the dispersion of thepresent invention to a substrate. Dipping the substrate into thedispersion, followed by air drying and sintering at the desiredtemperature with sufficient repetition to build the desired thickness.Spraying the dispersion onto the substrate is used to advantage forcovering large or irregular shapes. Pouring the dispersion onto thesubstrate is sometimes used. Painting the dispersion with brush orroller has been successfully employed. In addition, coatings may beeasily applied with metering bars, knives, or rods. Usually, thecoatings or films are built up to the thickness desired by repetitivedrying and sintering.

The type of substrate upon which the dispersion of the present inventionmay be applied can include such things as glass, aluminum foil,polytetrafluoroethylene tape, polytetrafluoroethylene sheets, metalsheets, or other polymer films or objects.

The substrate upon which the dispersion is to be deposited is cleaned ortreated in such a way as to assure uniform contact with the dispersion.The substrate can be cleansed by washing with a degreaser or similarsolvent followed by drying to remove any dust or oils from objects to beused as substrates. Metals should usually be acid etched, then washedwith a solvent to promote adhesion, if desired, unless the metal is newin which case degreasing is sufficient.

After being cleaned, the substrates may be pre-conditioned by heating orvacuum drying prior to contact with the dispersions and the coatingoperation. Temperatures and pressures in the following ranges arepreferably used: about 20 millimeters mercury at about 110° C. orthereabout is sufficient in all cases; however, mild heat is usuallyadequate, on the order of about 50° C. at atmospheric pressure.

After preparation, the substrates are coated with the dispersion by anyof several means including, but not limited to, dipping, spraying,brushing, pouring. Then the dispersion may be evened out using scrapingknives, rods, or other suitable means. The dispersion can be applied ina single step or in several steps depending on the concentration of thepolymer in the dispersion and the desired thickness of the coating orfilm.

Following the application of the dispersion, the dispersant is removedby any of several methods including, but not limited to, evaporation orextraction. Extraction is the use of some agent which selectivelydissolves or mixes with the dispersant but not the polymer.

These removal means should be employed until a uniform deposition ofpolymer is obtained and a continuous film is formed.

The dispersant removal is typically carried out by maintaining thecoated substrate at temperatures ranging from about 10° C. to about 110°C., with the preferred heating range being from about 20° C. to about100° C. The heating temperature selected depends upon the boiling pointof the dispersant.

Heating temperatures are customarily in the range of from about 20° C.to about 50° C. for 1,2-dibromotetrafluoroethane.

The pressures employed for the removal of the dispersant from the coatedsubstrate can range from about 20 mm mercury to about 760 mm mercurydepending on the nature of the dispersant, although pressures aretypically in the range of from about 300 mm mercury to about 760 mmmercury for 1,2-dibromotetrafluoroethane.

The forming of the coating or film can be carried out as part of thepolymer deposition and dispersant removal process or as a separate stepby adjusting the thermal and pressure conditions associated with theseparation of the polymer from the dispersant. If the dispersion is laiddown in successive steps, a continuous film or coating free frompinholes can be formed without any subsequent heating above ambienttemperature by control of the rate of evaporation. This can be done byvapor/liquid equilibrium in a container or an enclosure; therefore, thedispersant removal step can be merely a drying step or a controlledprocess for forming a coating or film. If the dispersant is removed asby flash evaporation a film will not form without a separate heatingstep.

After the dispersant has been removed, the residual polymer andsubstrate, as a separate step, is preferably subjected to a heat sourceof from about 150° C. to about 380° C. for times ranging from about 10seconds to about 120 minutes, depending upon the thermoplasticproperties of the polymers. The polymers having melt viscosities on theorder of 5×10⁵ poise at about 300° C. at a shear rate of 1 sec.⁻¹ asmeasured by a typical capillary rheometer would require the longer timesand higher temperatures within the limits of the chemical groupstability. Polymers with viscosities on the order of 1 poise at ambienttemperatures would require no further treatment.

The most preferred treatment temperatures are from about 270° C. toabout 350° C. and a time of from about 0.2 to about 45 minutes for themost preferred polymers for use in the present invention. Such polymersform thin continuous films under the conditions described above.

After two polymers have been applied to their respective substrate, theyare contacted with each other at a temperature, at a pressure and for atime sufficient to bond the two polymers together. Such temperatures areusually from about 150° to about 380° C. The pressures suitablepressures up to about 2000 psi. The times are from about 10 seconds toabout 120 minutes.

Thereafter, the removable substrates are removed. A variety of means canbe used to remove the substrate including chemically etching thesubstrate away, vaporizing the substrate, dissolving the substrate,peeling the substrate from the film, peeling the film from thesubstrate, and other physical or chemical means.

Composite films of varying layer thicknesses can be easily produced bythe methods and means described above. Such films are suitable asmembranes, when in their ionic forms, for use in electrochemical cells.They are particularly useful for the electrolysis of sodium chloridebrine solutions to produce chlorine gas and sodium hydroxide solutions.Membranes prepared according to the present invention have surprisinglygood current efficiencies when used in chlor-alkali cells.

EXAMPLES Example 1

A first polymer having an equivalent weight of about 850 was preparedaccording to the following procedure:

About 784 grams of CF₂ ═CFOCF₂ CF₂ SO₂ F was added to about 4700 gramsof deoxygenated water containing about 25 grams NH₄ O₂ CC₇ F₁₅, about18.9 grams of Na₂ HPO₄.7H₂ O, about 15.6 grams of NaH₂ PO₄.H₂ O andabout 4 grams of (NH₄)₂ S₂ O₈ under a positive pressure of about 192pounds per square inch gauge (psig) of tetrafluoroethylene at about 60°C. for about 88 minutes. The reactor was vented under heat and vacuum toremove residual monomers. The reactor contents was frozen, thawed, andvigorously washed to remove residual salts and soap.

About 30 grams of the first polymer was made into a dispersion usingabout 270 grams of 1,2-dibromotetrafluoroethane. The dispersion wascoated onto an aluminum foil and heated to about 300° C. for about 1minute. The coating and heating steps were repeated until a coatingabout 4 mils (about 102 microns) was achieved.

A second copolymer was then prepared. It was a copolymer of CF₂ ═CF₂ andCF₂ ═CFOCF₂ CF₂ SO₂ F having an equivalent weight of about 1144 wasprepared. The polymer was prepared according to the following procedure.About 784 grams of CF₂ ═CFOCF₂ CF₂ SO₂ F was added to about 4700 gramsof deoxygenated water containing about 25 grams NH₄ O₂ CC₇ F₁₅, about18.9 grams of Na₂ HPO₄.7H₂ O, about 15.6 grams of NaH₂ PO₄. H₂ O andabout 4 grams of (NH₄)₂ S₂ O₈ under a positive pressure of about 250pounds per square inch gauge (psig) of tetrafluoroethylene at about 60°C. for about 58 minutes. The reactor was vented under heat and vacuum toremove residual monomers. The reactor contents was frozen, thawed, andvigorously washed to remove residual salts and soap. After vacuumdrying, a dispersion was prepared by placing 56 grams of polymerprepared above in a laboratory-size single tier 290 revolutions perminute roller Norton Jar Mill with 168 grams of1,2-dibromotetrafluoroethane. The mixture was mixed in the ball millovernight at ambient temperature and at atmospheric pressure.

To the resulting soft paste about 300 additional grams of1,2-dibromotetrafluoroethane was added and the mill was rolled anadditional 3 hours. The resulting dispersion was found to contain about12.5 weight percent polymer. The mixture was coated onto a sheet ofaluminum foil approximately 38 microns thick by dipping the foil whichhad been formed into an envelope or pocket shape into the dispersion.The coated aluminum foil was allowed to air dry. Thus, the dispersantevaporated from the dispersion at ambient temperature.

The aluminum foil coated with the second copolymer was then heated atabout 300° C. in a muffle furnace for about 1 minute to fuse the polymerinto a more uniform film form.

The resulting thin film was found to be a continuous film and had athickness of about 0.5 mils (about 12.7 microns).

The dipping and heating process was repeated 5 times until a 2.5 mil(about 63.5 microns) thick second polymer film was built up.

Then the two coated foils above were pressed together, coated side tocoated side, at about 300° C. at a pressure of about 400 psig for about3 minutes. The resulting two layer composite membrane was hydrolyzed ina 25 weight percent aqueous sodium hydroxide solution. It was then runsatisfactorily in a chlor-alkali test cell.

We claim:
 1. A method for forming polymer composite films using aremoval substrate comprising:(a) forming a first dispersion of a firstperfluorinated polymer containing sites convertible to ion exchangegroups and a dispersant having: a boiling point less than about 110° C.;a density of from about 1.55 to about 2.97 grams per cubic centimeter;and a solubility parameter of from greater than about 7.1 to about 8.2hildebrands; (b) depositing the first dispersion onto a first removablesubstrate; (c) removing the first dispersant from the first dispersion,thereby forming a first film; (d) forming a second dispersion of asecond perfluorinated polymer containing sites convertible to ionexchange groups and a second dispersant having: a boiling point lessthan about 110° C.; a density of from about 1.55 to about 2.97 grams percubic centimeter; and a solubility parameter of from greater than about7.1 to about 8.2 hildebrands; (e) depositing the second dispersion ontoa second removable substrate; (f) removing the second dispersant fromthe second dispersion, thereby forming a second film; (g) bonding thefirst film to the second film; and (h) removing the first and the secondsubstratewherein the first and the second dispersants are independentlyrepresented by the general formula:

    XCF.sub.2 --CYZX'

wherein: X is selected from the group consisting of F, Cl, Br, and I; X'is selected from the group consisting of Cl, Br, and I; Y and Z areindependently selected from the group consisting of H, F, Cl, Br, I andR'; R' is selected from the group of perfluoroalkyl radicals andchloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.
 2. Themethod of claim 1 wherein the first and the second perfluorinatedpolymers are independently selected from the group of copolymers formedfrom a first type of monomer and a second type of monomer;wherein thefirst type of monomer is represented by the general formula:

    CF.sub.2 =CZZ'                                             (I)

where: Z and Z' are independently selected from the group consisting of--H, --Cl, --F, or CF₃ ; and the second monomer is represented by thegeneral formula:

    Y--(CF.sub.2).sub.a --(CFR.sub.f).sub.b --(CFR'.sub.f).sub.c --O--[CF(CF.sub.2 X)--CF.sub.2 --O].sub.n --CF=CF.sub.2   (II)

where: Y is selected from the group consisting of --SO₂ Z, --CN, --COZand C(R³ _(f))(R⁴ _(f))OH; Z is I, Br, Cl, F, OR, or NR₁ R₂ ; R is abranched or linear alkyl radical having from 1 to about 10 carbon atomsor an aryl radical; R³ _(f) and R⁴ _(f) are independently selected fromthe group consisting of perfluoroalkyl radicals having from 1 to about10 carbon atoms; R₁ and R₂ are independently selected from the groupconsisting of H, a branched or linear alkyl radical having from 1 toabout 10 carbon atoms or an aryl radical; a is 0-6; b is 0-6; c is 0 or1; provided a+b+c is not equal to 0; X is Cl, Br, F or mixtures thereofwhen n>1; n is 0 to 6; and R_(f) and R'_(f) are independently selectedfrom the group consisting of F, Cl, perfluoroalkyl radicals having from1 to about 10 carbon atoms and fluorochloroalkyl radicals having from 1to about 10 carbon atoms.
 3. The method of claim 2 wherein the first andthe second perfluorinated polymers independently include a third type ofmonomer wherein the third type of monomer is one or more monomersrepresented by the general formula:

    Y'--(CF.sub.2).sub.a' --(CFR.sub.f).sub.b' --(CFR'.sub.f).sub.c' --O--[CF(CF.sub.2 X')--CF.sub.2 --O].sub.n' --CF=CF.sub.2 (III)

where: Y' is F, Cl or Br; a' and b' are independently 0-3; c' is 0 or 1;provided a'+b'+c' is not equal to 0; n' is 0-6; R_(f) and R'_(f) areindependently selected from the group consisting of Br, Cl, F,perfluoroalkyl radicals having from about 1 to about 10 carbon atoms,and chloroperfluoroalkyl radicals having from about 1 to about 10 carbonatoms; and X' is F, Cl, Br, or mixtures thereof when n'>1.
 4. The methodof claim 1 wherein the boiling point of the first and the seconddispersant is from about 30° C. to about 110° C.
 5. The method of claim1 wherein the density of the first and the second dispersant is fromabout 1.55 to about 2.2 grams per cubic centimeter.
 6. The method ofclaim 1 wherein the solubility parameter of the first and seconddispersant is from greater than about 7.1 to about 7.5 hildebrands. 7.The method of claim 1 wherein the density of the first and seconddispersant and the density of the first and second polymer are both fromabout 1.55 to about 2.2 grams per cubic centimeter.
 8. The method ofclaim 1 wherein X and X' are Br.
 9. The method of claim 1 wherein X andX' are Cl.
 10. The method of claim 1 wherein the first and the secondpolymers are present in the first and the second dispersions at aconcentration of from about 0.1 to about 50 weight percent.
 11. Themethod of claim 1 wherein the first and the second polymers are presentin the first and the second dispersions at a concentration of from about0.3 to about 30 weight percent.
 12. The method of claim 1 wherein thefirst and second removable substrates are made of aluminum.
 13. Themethod of claim 1 wherein the first and the second substrates areindependently removed by dissolving with a solvent for the substrate.14. The method of claim 1 wherein the first and the second substratesare individually removed by an alkaline solution.
 15. The method ofclaim 1 including heating the coated first and second substrates to fusethe first and second polymers into films prior to removing thesubstrate.